20．Pick up a pencil．Lift your foot．Raise your arm．These directions may sound simple，but many parts of your body must work together for you to make those movements．Your joints are parts of your body that help your bones move．With little effort，joints move bones in different directions．But some people's joints do not work well because of illness or injury．Their joints sometimes have to be replaced with artificial ones．
Joints look simple．You might think that scientists would have no trouble making artificial joints that work as well as real joints．After over a century of trying，scientists are just beginning to find ways of making artificial joints that work well．
What problems have to be solved in building an artificial joint？First，some joints have to be strong enough to support most of a person's weight．The first artificial hip-joint，built in 1891，failed because it could not hold up a person's weight．
Second，joints have to be flexible to allow movement in different directions．
Third，joints must be made of materials that the body will not reject．Usually，the body reacts to foreign substances by producing cells that attack and destroy the substances．To the body an artificial joint is a foreign substance that should be destroyed．
Scientists have finally found ways to solve these problems．Nowadays the artificial knee-joint is made of a special metal and plastic．The metal is a combination of silicon （硅） and stainless steel．The metal is strong，so it can support the weight of the body．The plastic is a flexible material．It allows the joint to move easily．Medicines have been discovered that will prevent the body from rejecting the artificial joint．People who have lost the use of a joint can get an artificial joint that works almost as well as the original one．
81．Which parts of our body help our bones move？（No more than 2 words）The joints
82．How many years have scientists been working to find ways of making artificial joints that work well？（No more than 4 words）Over a century
83．How does our body react to foreign substances？（No more than 9 words）It produces cells that attack andd estroy the substances
84．Why can the special metal and plastic used for artificial joints？（No more than 10 words）Because the metal is strong and the plastic is flexible．
分析 本文講述了科學家研制出假關節(jié)，以來幫助那些關節(jié)受傷的人能自由移動，以及在制作假關節(jié)時需要注意的事情．
解答 81．The joints．/Our joints．細節(jié)理解題．Your joints are parts of your body that help your bones move這句話告訴我們?nèi)梭w關節(jié)部位幫助人身體移動．可以直接找到答案．
82．Over a century．/More than 100 years．細節(jié)理解題．在第二段After over a century of trying，scientists are just beginning to find ways of making artificial joints that work well．這句話里的時間狀語over a century，說明科學家開始開始研制的時間．
83．It produces cells that attack and destroy the substances．/By producing cells that attack and destroy the substances．細節(jié)概括退．在注意事項的第三點里，Usually，the body reacts to foreign substances by producing cells that attack and destroy the substances．這句話告訴我們身體對外來物質(zhì)的抵抗主要因為這些物質(zhì)會產(chǎn)生一些有攻擊性的細胞．
84．Because the metal is strong and the plastic is flexible．細節(jié)概括題．最后一段The metal is strong，so it can support the weight of the body．The plastic is a flexible material．可以得知，金屬堅硬，塑料柔軟，是作為人造關節(jié)的首選材料．
點評 閱讀表達，要求學生在閱讀理解的基礎上，根據(jù)短文后的題目，在文中找到相關的內(nèi)容，并用簡潔、準確的句子，將理解的內(nèi)容，書面表達出來．做題時，注意表達的內(nèi)容不要超出規(guī)定的字數(shù)．
